I don't think he's ever कहा it as a stand-alone quote, but he's कहा it in a sentence. In What's New Scooby-Doo, the episode "There's No Creature Like Snow Creature", he says "I don't like the sound of that 'hmmm'." This instance is also spoken द्वारा Casey Kasem, the original Shaggy's voice, if that matters to you.
Also in "Coast to Ghost", one of the episodes in the 13 Ghosts collection, someone says "Hmm, I don't like the sound of that music, या of that name, Professor Fantazmo." I'm not sure it's Shaggy, though.
